FIELD OF THE INVENTION
This invention relates to rate sensors and more particularly to an accurate analog angular rate sensor which is implemented without mechanical brushes.
Prior art devices include: brush type direct current (DC) tachometers; resolver or hall effect devices used in conjunction with two phase brushless motors; and differentiating position data obtained from resolvers. Some prior art devices are disclosed in U.S. Pat. Nos. 3,858,109 issued Dec. 31, 1974 to Sam P. Niden; 4,060,799 issued Nov. 29, 1977 to Donald P. Jones; 4,199,800 issued Apr. 22, 1980 to James G. Weit; 4,755,751 issued Jul. 5, 1988 to Glen Ray; 4,901,566 issued Feb. 20, 1990 to Antoine Boetsch; 4,962,331 issued Oct. 9, 1990 to Charles G. Smith.
A disadvantage of brush type DC tachometers is that brushes tend to be unreliable. This is particularly true in space environment use such as space station mechanisms, etc.
One disadvantage of resolver of hall type devices used in conjunction with two phase brushless motors is that the hall devices and off the shelf motors do not always produce clean and accurate sinusoidal waveforms. This directly translates into rate error. On the other hand resolvers are usually designed for very high accuracy. A second disadvantage of utilizing a brushless motor (or a DC brush type tachometer) is that in a redundant system if the motor or DC brush type tachometer happens to fail with a shorted winding their associated back electromagnetic field (emf) is shorted. This results in an additional torque which the redundant system (if mounted to a common output shaft) will have to overcome.
A disadvantage of differentiating position data from resolvers is that electronic differentiating tends to be very noisy. Also to accomplish an accurate rate signal the sinusoidal signals from the resolver must be selectively switched to the input of the differentiator depending on the angle of rotation. This switching also has a tendency to be noisy. This method also has limited accuracy at low rates.
It is an object of the present invention, therefore, to provide a brushless resolver rate assembly to overcome the disadvantages of the above mentioned prior art devices.